Diffusion coating for metals



489,064 6 Cfaims. (Cl. 117-414) This invention relates to a method of providing a chromium-containing diffusion coating on a base metal or alloy, and more particularly to a method for providing a chromium-nickel diffusion coating on a ferrous metal.

Industrial and commercial uses of metals at high temperatures and in corrosive environments have continually increased, and the number of operations requiring metals with improved special properties is steadily rising. For a great many uses, it is essential that metal parts resist oxidation and other chemical surface reactions at high temperatures and abnormal conditions. Further, it is frequently desirable to have metal alloys of variable composition, for example, a very hard, corrosion-resistant surface, and a base having good working characteristics-properties which frequently are not found with an alloy of uniform composition. Metals having corrosionresistant surfaces at high temperatures are required, for example, for the following items: furnace parts, turbine blades, petroleum refinery equipment, jet engine sheet metal work, superheater tubes, and chemical plant equipment; the list of applications for ferrous metals with corrosion-resistant surfaces is virtually limitless. Thus, a process which can efficiently improve the temperature corrosion properties of metals, particularly ferrous metals, is of considerable interest.

Diffusion coatings on metals and alloys have been used to change one or more metallurgical properties, for example, to produce an atmospheric oxidation-resistant surface, a surface resistant to the action of specific chemicals, and a wearand oxidation-resistant surface. It may be convenient to first make a particular piece from an alloy which is in easily formable condition, and then add the diffusion layer coating to obtain the desired property. A diffusion layer, which usually involves formation of an intermetallic compound or a solid solution of the coating metal with the base metal, has distinct advantages over a simple plating layer.

Chromium is widely used to improve the surface properties of ferrous metals, and also of refractory metals, such as molybdenum. A diffusion coating of chromium is generally applied by so-called pack cementation methods, wherein a base metal part is placed in a high temperature retort containing a powdered chromium composition, an inert filler, and a halide salt. Upon heating, chromium halide vapors form which deposit upon the part and result in the formation of a chromium diffusion coating. Among the drawbacks of such processes are difficulties in coating complex shapes, particularly objects with hidden recesses or bores. The parts must be cleaned prior to use, and time is consumed in packing the part and the chemicals in the retort, and then later in unloading the assembly. These operations increase the cost of such processes. Further, pack methods have been generally restricted to rather small parts and are not utilizable for large, finished articles because of equipment limitations.

3 ,377,195 Patented Apr. 9, 1968 The pack methods are also not adapted to the formation of chromium-containing alloys of very high corrosion resistance, for example chromium-nickel, which on a ferrous metal base would form an Fe-Cr-Ni alloy, equivalent to a stainless steel or superalloy cladding on a low alloy or carbon steel base. By being able to vary the relative proportions of the elements present in the chromium-nickel coating, ferrous alloy metals whose surfaces have unique properties are obtainable. Thus special ferrous alloys may be tailor-made to meet specific requirements.

In addition to the pack cementation methods, it has also been proposed for certain specific applications to form certain diffusion coatings on particular base metals by placing the base metal in a containing therein the metal to be used as the coating material. Such processes are highly specific and unique and require the favorable concatenation of many fre quently conflicting variables in order to be of more than be an exotic or unduly expensive material, because dragout losses during the process would render its use commercially prohibitive. Further, because high temperatures are required for obtaining the molten bath and for effective rates of deposition of the coating metal, safety of operation of the bath at these elevated temperatures is an important consideration. In addition, the molten metal transfer agent must have a suitable melting point and vapor pressure, particularly at temperatures where effective diffusion rates occur. Further, metal transfer agent must show selective solubility both with respect to the base metal being coated and also with respect to the solubility of the coating material and its rate of deposition from the molten bath. Where a conjoint coating is proposed, e.g., chromium-nickel, the problems involved are further complicated. Thus operating conditions that may be satisfactory for obtaining a suitable layer of one coating material within a given time may be wholly in appropriate for obtaining a coating of a second material or for obtaining a composite coatingaThus the obtaining of a chromium-nickel coating, particularly one wherein the relative proportions of the chromium and nickel in the coating may be varied, is particularly difficult to achieve under commercially feasible conditions, particularly Where close reproducibility of coating thicknesses and compositions is required.

In US. Patent 3,184,331, the use of molten calcium, barium, strontium, and magnesium baths are proposed for diffusing chromium, nickel, manganese and cobalt. While such a process has utility for certain applications, it suffers from the disadvantages of requiring unduly high operating temperatures and of varying reproducibility of coatings, particularly where conjoint deposition of nickel and chromium is desired. Where lower temperatures are employed for the process, the chromium-nickel coating obtained is generally nickel-poor.

In US. Patent 3,186,865 there is disclosed a method of forming a chromium-containing diffusion coating on a dissimilar base metal by placing the base metal in a molten lithium bath containing chromium, and optionally nickel, dissolved therein, and maintaining the base metal in the bath until a chromium or chromium-nickel diffusion layer of desired thickness is obtained.

While the process described in US. Patent 3,186,865 leads to high quality chromium and chromium-nickel diffusion coatings, further significant economic and other advantages may be obtained by the present invention which eliminates certain disadvantageous features present in the patented process.

The present invention provides a process with improved advantages with respect to economy of operation, lower operating temperature, increased safety, and improved coatings compared with known processes which make this process of significant commercial interest.

Accordingly, an object of the present invention is to provide an improved method of forming a chromiumcontaining diffusion coating on base metals and alloys.

Another object is to provide a method of applying chromium upon a ferrous metal base to improve the oxidation and corrosion resistance of such base metals.

Another object is to provide a. method particularly suitable for the conjoint deposition of a chromium-nickel coating on a ferrous base metal.

Another object is to provide a method wherein the chromium-containing diffusion material may be applied to all surfaces of a base metal independent of the configuration or shape of the base metal.

Still a further object is to provide a chromium-containing surface diffusion coating of a graded alloy of variable composition on a ferrous metal in order to increase its resistance to high-temperature surface corrosion.

In accordance with the present invention, a chromiumalloyable base metal is provided with a chromium-containing diffusion coating by placing the base metal in a composite molten calcium-lithium bath containing chromium values dissolved therein, and then maintaining the base metal in the bath for a sufficient time for the dissolved chromium to diffuse into the base metal to form a diffusion coating containing chromium on the base metal.

Because of the need and commercial importance of protecting ferrous base compositions, and because of the commercial acceptability of chromium-containing coatings in this regard, a particularly preferred and commercially significant aspect of this invention is the providing of chromium and chromium-nickel diffusion coatings on a ferrous base metal from a molten calcium-lithium bath. However, for certain other specialized applications, other donor materials, e.g., cobalt, may be codeposited with chromium, and other base metals may be used, such as the refractory metals and those selected from groups IV-B and V43, and the first triad of group VIII of the Periodic Table.

It is an essential feature of this invention that a composite molten calcium-lithium bath be used from which the chromium-containing diffusion coating is deposited. This molten bath, wherein the calcium and lithium function as active transfer agents, essentially contains between and 60 weight percent lithium, with the balance calcium, these percentages being taken without reference to the coating materials present in the bath. On an atom percent basis, the molten calcium-lithium bath would therefore correspond to a lithium range between 38 and 90 atom percent and a corresponding calcium range between 62 and 10 atom percent. Particularly suitable and preferred results are obtained in a molten bath containing :5 weight percent lithium.

Somewhat surprisingly, the use of the composite calcium-lithium molten bath of this invention from which a chromium-containing diffusion coating is deposited provides those essential advantages inherent in the use of either the lithium or calcium bath alone, while at the same time eliminating the disadvantages associated with the use of either single bath. In addition, the ability to vary the relative proportions of the two active transfer agents constituting the molten bath, with the further ability to vary the relative amounts of dissolved coating material, results in considerable flexibility in the obtaining of a range of conjointly deposited diffusion coatings.

There will usually be actual diffusion of the chromium and chromium-nickel into the base metal, with resulting fomation of a solid solution, alloy, or inter-metallic compound that is chromium rich at the surface. Diffusion layers with graded boundary layers varying from 0.1 mil up to 60 mils may be obtained, although diffusion coatings typically varying from 0.5 mil to 10 mils are preferred. The diffusion of the coating material into the base metal accounts for the changed metallurgical properties of the resulting article, for instance improved mechanical and chemical properties. The resulting diifusion layer has a number of distinct advantages over a simple plating layer, primarily due to the fact that the coating is graded in composition. There is no sharp interface, which is beneficial in preventing spalling or breaking of the coating. With a ferrous metal base, a chromium-iron or chromiumnickel-iorn alloy is formed. The oxidation and corrosion resistances of such diffusion-coated ferrous metals are very great, while the cost is considerably less than that of stainless steel.

While the mechanism of the several steps of the process, such as solution, transport, deposition and diffusion of the chromium into the base metal, as well as the formation and nature of the alloy diffusion layer (e.g., intermetallic compound or solid solution) is not fully understood, the use of the molten calcium-lithium bath as transfer agent is an essential feature of this process. Because of the very high negative free energy of formation of calcium oxide, the presence of the calcium component of the bath results in the maintaining of extremely clean oxide-free surfaces on the base metal, initially and throughout the coating operation. The use of calcium-lithium enables operation at a lower temperature than may be employed with either molten metal alone while still obtaining a satisfactory rate of deposition. Thus, while lithium melts at a lower temperature than does the calcium-lithium bath, suitable rates of deposition with the lithium bath alone are obtainable only at elevated temperatures of the order of 2000" F., at which temperatures lithium has a relatively high vapor pressure. Further, because of the lower cost of calcium compared with lithium, the use of the composite molten bath is particularly economical where dragout losses occur, compared with a lithium bath alone.

It has further been found, where molten baths are used for diffusion coating, that the time required for cleaning of the coated article to remove adherent impurities and bath constituents is of significant concern in the commercial utilization of the process. Water is ordinarily preferred for use as an inexpensive and convenient cleaning agent. Where the molten bath is lithium alone, the reaction of the adherent lithium with the water proceeds at a very rapid rate and presents safety hazards. With a calcium bath alone, an adherent layer of calcium hydroxide tends to form on the coated article, and clean ing is troublesome and proceeds at a slow rate. By use of the composite molten calcium-lithium bath, the coated article may be readily cleaned with water at a rapid rate consistent with safety requirements, and no adherent layer of calcium hydroxide forms to interfere with the cleaning operation.

The calcium-lithium bath is protected from oxidation during processing by maintaining an inert environment thereover, i.e., a non-oxidizing one, such as may be ob tained under vacuum or by maintaining an inert gaseous atmosphere provided by a noble gas such as helium or argon.

The base metal may be any metal which does not dissolve in the molten composite calcium-lithium bath at an appreciable rate for the time of immersion used. Of particular interest are the ferrous metals, in view of their wide usage, and nickel and cobalt compositions (i.e., the first triad of group VIII). Also of interest are J the refractory metals, for example various group IV-B metals such as titanium, group V-B metals such as niobium, and group VIB metals such as molybdenum and tungsten. Ordina'rly there would be no interest in coating chromium per se with a protective coating of chromium. Reference made herein to the various groups of elements refers to the Periodic Chart of the Elements as shown in Handbook of Chemistry, th edition, N. A. Lange, editor, McGraw-Hill Book Company, New York, 1961, pages 56-57.

However, because of the commercial importance of the chromizing of ferrous base metals, the details of the present invention will be exemplified by reference thereto. The calcium-lithium bath must, of course, be maintained in a molten condition. Where the bath contains 10 weight percent lithium, the bath will be molten at a temperature of about 760 F. Where the lithium content of the bath is increased to the upper limit of 60 weight percent, the composite bath will be molten at a very much lower temperature, about 450 F. This low melting point allows for greater ease in removing adherent bath materials from base metals coated at much higher temperatures of operation. For certain applications, higher temperatures of operation may be preferred or required in order to obtain suitable rates of deposition. A preferred range for depositing chromium coatings on ferrous base metals is between 1400 and 1800 F. This upper limit, which is below that generally required for lithium baths alone, is ordinarily dictated by the capabilities of the container materials use in the process since the corrosive activity of the molten bath increases with temperature. Within the preferred operating temperature range of 1600l800 F., for a molten bath containing 25 weight percent lithium, there is little if any loss from an open bath due to evaporation within this temperature range and no need to provide for pressurization. Accordingly, a higher upper temperature may be used where desired.

The chromium values dissolved in the molten calciumlithium bath will generally constitute from 1 to 20 percent by weight of chromium, based on the calcium-lithium content. The chromium values are preferably dissolved in the calcium-lithium bath either in the form of chromium metal, which requires pulverizing the chromium into suitable powdered form for ease of solution, or in the form of chromium oxide (Cr O which is generally available in the form of powder and readily dissolves in molten calcium-lithium. Where chromium oxide is added, approximately 1.5 to 30% by Weight of chromium oxide will be added to give the equivalent amount of chromium metal. Nickel may be additionally added to the bath in the form of metal powder or shot, or in the form of its oxide, in amounts between 1 and 30 weight percent. Because of the high negative free energy of formation of calcium oxide, any chromium oxide or nickel oxide is almost immediately reduced to chromium or nickel, respectively. The amount of chromium and nickel values dissolved in the bath will depend to some extent on the surface area of the specimen to be coated and on the relative proportions desired of the coating with respect to a codeposited chromium-nickel coating. It should be noted that the composition of a codeposited chromiumnickel coating may be varied by altering the composition of the molten calcium-lithium bath, the relative amounts of nickel and chromium dissolved in the bath, and the temperature at which the coating operation is performed.

Diffusion times varying from 30 minutes to 10 hours may be employed in this process, a coating of approximately 0.5 mil being obtained in about 1 hour and a coating of about 1.5 mils being obtained in about 3 hours under typical conditions. It will of course be recognized that the rate of deposition of the diffusion coating is not a linear function but depends upon the relationship and interaction of many factors not fully understood. In actual practice, the rate of diffusion appears to vary logarithmically with time. The time re- 8 quired for the coating of the base metal will further be a function of the temperature of the bath and the concentration of the chromium as well as the coating thickness desired.

Agitation of the molten calcium-lithium bath will be another factor affecting the time required to form a satisfactory diffusion coating of chromium of a given quality 'and thickness on the ferrous base metal. This agitation may be conveniently accomplished by mechanical agitation of the reaction chamber and its contents, for example by use of rotating and see-saw type capsules. Alternatively, the capsule or container and its contents may be vibrated, or the contents only may be agitated, or the liquid only may be agitated.

A batch, semi-continuous, or continuous diffusion coating process may be used in the practice of this invention. Conveniently, for all processes any container non-reactive with a calcium-lithium may 'be used to hold the bath. Convenient materials are alloy steel, stainless steel, niobium, and tantalum. In batch processing, specific capsule configurations may also be used to support some of the constituents, to provide a reservoir, or to house unusually shaped specimens. The specimen to be coated may be placed into the bath loose, suspended on wire, encased in a screen envelope or passed through the bath, in the case of semicontinuous and continuous processing.

Various types of furnaces may be used to house the batch-processing capsule, the simplest being a static furnace which is temperature controlled to maintain an isothermal environment for the capsule. In other application, a tube furnace is used inclined at 30 from the horizontal within which the capsule is rotated about its longitudinal axis at about 15 rpm. Additionally, a rocking motion may be added to the rotating motion. For this type of application, a tube furnace is used in which the capsule is rotated at 15 rpm. while the furnace and capsule are both rocked through i 30 from the horizontal about a mid-point pivot.

An open vessel bath may be used for the batch, semicontinuous, or continuous process. The temperature is maintained by a furnace around the vessel containing the bath. A recirculating argon environment is maintained over the bath solution to prevent any oxidation occurring.

The following examples are offered to illustrate the scope, practice, and advantageous features of this invention in greater detail, and are not to be construed as limitations thereof.

Example 1.-Chromium-nickel diffusion coating A capsule of type 304 stainless steel tubing was filled with a bath of 25 weight percent lithium, weight percent calcium, to which Was added 10 weight percent chromium and 10 weight percent nickel, based on the calcium-lithium weight. Targets of a mild steel (A181 1010) were placed in the bath. The capsule was heated at 1 600" F. for 5 hours while being agitated by a rocking and rotating motion. A Weight gain of 6.37 nag/cm. and a case depth of 1 mil were obtained. Chemical analysis showed this case to contain 19.34 Weight percent chromium and 34.32 weight percent nickel.

Example 2.Deposition of chromium diffusion coating Example 3.Comparison of Ca-Li bath With Ca and Li baths Six runs were performed using the apparatus and under the general conditions used for Example 1. The

results obtained are shown in Table I.

TAB LE I G homieal Analysis, Bath Donor Temp., Time, Wt. Gain, Case wt. percent F. hrs. Inge/cm. (in) Gr Ni Chromium, nickel 2- 2, 000 7 11. 0016 13. 6 -do 1,650 3.3 .0009 24.9 0 1,600 5 6.4 .001 10.3 Chromium" 5 0018 23. 9 Calcium .do 1, 650 5 3. 0 0011 19. 5 Calcium/Lithium 1 -d0 1, 600 5 3. 4 0011 23. l

1 wt. percent Li.

Example 4.-'Chromized mild steel specimens Mild steel specimens were chromized in a calciumlithium bath containing 25 weight percent lithium. The samples were run at a temperature of 1700 F. for times ranging from 1 hour to 8 hours. The following results were obtained:

Exposure Wt. gain Time, hrs.: mgJcrn. 1 1.81 2 2.59 4 3.62 8 5.11

Specimens exposed for different times were subjected to salt spray corrosion tests (5% NaCl solution at 95 F. for 24 hours). The specimen coated for 8 hours withstood four 24-hour exposure cycles. The specimens coated for 1 and 2 hours showed small pinhole defects on their faces and edges at the end of the salt spray corrosion test. The 4-hour coated specimens showed defects predominately limited to the specimen edges.

Specimens were also evaluated for corrosion resistance by immersion in 1-0 percent nitric 'acid at 90 F. for 24 hours. The 1-hour and 2-hour coated specimens showed iron loss and blistering of the coating. The 4-hour and 8-hour specimens were virtually intact and showed no Weight loss.

It will of course be understood that many variations are possible in the practice of this invention, depending upon the coating thickness desired, the base metal used, the chromium material from which the dissolved chromium values are obtained, as well as the interrelationship between bath composition, bath concentration, agitation of bath, bath temperature, and coating time, and these variants are therefore considered to lie within the practice of this invention. Accordingly, the scope of this invention should be determined in accordance with the objects thereof and the appended claims.

I claim:

1. The method of forming a diffusion coating containing chromium on a chromium-alloyable base metal selected from the class consisting of refractory metals and ferrous metals which comprises placing the base metal in a molten calcium-lithium bath containing from 10 to weight percent lithium based on the calcium-lithium content maintained under an inert environment, said bath containing chromium values dissolved therein equivalent to from 1 to 20 weight percent of chromium based on the calcium-lithium content, and maintaining said base metal in said bath at a temperature between about 1400 F. and 1800 F. until a diffusion coating containing chromium is obtained on said base metal.

2. The method of claim 1 wherein said molten bath contains between 20 and 30 weight percent lithium based on the calcium-lithium content.

3. The method of claim 1 wherein sai-d molten bath additionally contains nickel values dissolved therein equivalent to between 1 and 30 "weight percent nickel based on the calcium-lithium content and the formed diffusion coating additionally contains nickel.

4. The method of claim 3 wherein said chromium and nickel values are provided by respectively dissolving metallic chromium and metallic nickel in said molten calcium-lithium bath.

5. The method of claim 1 wherein said chromium- :alloyable base metal is a ferrous metal.

6. The method of claim 3 wherein said chromiumalloyable base metal is a ferrous metal.

References Cited UNITED STATES PATENTS 3,184,292 5/1965 Argyriades et al. 1l7-1l4 X 3,184,331 5/1965 Carter 117-114 3,186,865 6/1965 Page 117-119 3,261,712 7/1966 Carter 117114 ALFRED L. LEAVITT, Primary Examiner. I. R. BATTEN, 3a., Assistant Examiner 

1. THE METHOD OF FORMING A DIFFUSION COATING CONTAINING CHROMIUM ON A CHROMIUM-ALLOYABLE BASE METAL SELECTED FROM THE CLASS CONSISTING OF REFRACTROY METALS AND FERROUS METALS WHICH COMPRISES PLACING THE BASE METAL IN A MOLTEN CALCIUM-LITHIUM BATH CONTAING FROM 10 TO 60 WEIGHT PERCENT LITHIUM BASED ON THE CALCIUM-LITHIUM CONTENT MAINTAINED UNDER AN INERT ENVIROMENT, SAID BATH CONTAINING GROMIUM VALUES DISSOLVED THEREIN EQUIVALENT TO FROM 1 TO 20 WEIGHT PERCENT OF CHROMIUM BASED ON THE CALCIUM-LIGHIUM CONTENT, AND MAINTAINING SAID BASE METAL IN SAID BATH AT A TEMPERATURE BETWEEN ABOUT 1400*F. AND 1800*F. UNTIL A DIFFUSION COATING CONTAINING CHROMIUM IS OBTAINED ON SAID BASE METAL. 